This invention relates to quantum-splitting fluoride-based phosphors. More particularly, the phosphors are fluorides doped with Pr3+. This invention also relates to method of producing and blends comprising such phosphors.
The conversion of a single ultraviolet (“UV”) photon into two visible photons with the result that the quantum efficiency of luminescence exceeds unity is termed “quantum splitting.” Quantum splitting materials are very desirable for use as phosphors for lighting applications, such as fluorescent lamps. A suitable quantum splitting phosphor can, in principle, produce a significantly brighter fluorescent light source due to higher overall luminous output because it can convert to visible light the part of UV radiation that is not absorbed efficiently by traditional phosphors currently used in commercial fluorescent lamps. Quantum splitting has been demonstrated previously in certain fluoride- and oxide-based materials. A material comprising 0.1% Pr3+ in a matrix of YF3 has been shown to generate more than one visible photon for every absorbed UV photon when excited with radiation having a wavelength of 185 nm. The measured quantum efficiency of this material was 140%, and thus greatly exceeded unity. However, this material did not show sufficient stability to permit its use as a phosphor in fluorescent lamps because of its tendency to react with mercury vapor. Recently, lanthanum magnesium borate and strontium magnesium aluminate, both activated with Pr3+, have been shown to exhibit quantum-splitting behavior. Emission spectra of these materials exhibit a large peak at wavelength of about 405 nm, which is characteristic of quantum splitting. However, these materials still exhibit a considerable emission in the UV wavelength range of less than 350 nm. This part of the emission reduces the overall visible light output that otherwise can be higher.
Recently, light sources based on mercury-free discharges have become important. A large portion of the emission of these discharges is typically in the vacuum UV range (“VUV”), which emission can be converted very efficiently by quantum-splitting phosphors to visible light. Therefore, the quest for improved quantum-splitting phosphors continues. It is also desirable to provide more energy-efficient light sources using quantum-splitting phosphors having higher quantum efficiency. It is further desirable to provide environmentally friendly methods for producing such phosphors.